Efficacious elimination of crystal violet pollutant via photo-Fenton process based on Gd(2−x)La(x)Zr2O7 nanoparticles

The photo-Fenton process is an appropriate method of the Advanced Oxidation Process that is used in the photocatalysis of organic dyes like crystal violet (CV). La3+ ion substituted gadolinium zirconium oxide Gd(2−x)La(x)Zr2O7 nanopowders (x = 0.1, 0.2, 0.3, and 0.5) have been successfully prepared by using sol–gel auto-combustion method to be used for the efficient photocatalysis of CV with photo-Fenton process. The well-crystallized defect-fluorite, structured with space group: Fm-3m, was detected using X-ray diffraction analysis. The lattice parameters were found to increase with the evaluated La3+ ion concentration. The grain size of the synthesized powders increased with the increase in La3+ ion content. The SAED patterns depicted fluorite structured fluorite. UV/Vis. spectrophotometer was used for the determination of band gap energy of Gd(2−x)La(x)Zr2O7 nanopowders which increased with increasing La3+ ion content. It was found to enhance from 4 to 3.6 eV. The visible spectrophotometer was used for determining unknown concentrations during the photocatalysis process to assure the effectiveness of the process. Overall, results illustrate that the photo-Fenton reaction on Gd(2−x)La(x)Zr2O7 performed excellently in removing crystal violet (CV). The photo-remediation ratio of CV reached 90% within 1 h.

On the other hand, better oxidants, microwave catalysis, photocatalysis, membrane technology, and advanced oxidation processes (AOPs) seem promising for CV decolorization [20][21][22] . The fundamental flaw of physical treatment is that it merely moves dyes from a liquid to a solid form, which is difficult to clean. As a result, for degrading such pollutants, chemical treatment employing AOPs, particularly heterogeneous photocatalysis, has garnered interest 23 .
According to Wang et al. 24 , metal oxides in heterogeneous AOPs produce powerful, non-selective hydroxyl radicals (HO • ) that break down a range of organic contaminants into short-chain aliphatic acids, which can be more readily eliminated 25 . In UV-visible light, the electron-hole pair process is necessary to introduce intermediate organic molecules that may completely mineralize at the surface of metal oxide, producing green end products. Since visible light is less expensive than UV light from an economic perspective, it has been extensively studied how new nano-sized photocatalysts respond to it.
In previous works, lots of materials were introduced in the photocatalysis process of dyes contained in wastewater. Nanosphere of TiO 2 , Mn-doped and PVP-capped ZnO NPs, Ag-modified Ti-doped-Bi 2 O 3 , ZnS NPS, CeO 2 -TiO 2 nanocomposite, AgBr-ZnO nanocomposite, and grafted sodium alginate/ZnO/graphene oxide have all been utilized to decolorize CV 26 . Liu et al. 27 used PM-Fe/Ni/H 2 O 2 in the degradation of crystal violet with a Fenton like reaction by Fenton-like reaction. Oladipo et al. 28 prepared Octadecylamine capped cadmium nanoparticles CdS to be used in the CV photodegradation process. ZnO/CNA was also prepared for the degradation of CV by Messaoudi et al. 29 and lots of materials are used for the degradation of CV as also other organic dyes.
The performance of the aforementioned oxides will next be evaluated, and the results of the present investigation will be contrasted. In this work, the incorporation of lanthanum with Gd 2 Zr 2 O 7 was discussed to enhance the value of the energy gap value to be appropriate for the photocatalysis process, especially by the Fenton-like method. The reason why we choose pyrochlore is its good ability in the photodegradation of organic dyes due to its chemical composition that distinguishes it from other compounds. This feature qualifies it to be a good photocatalyst. Since many types of pyrochlore were used before in the analysis processes, we used pyrochlore with new elements Gd (2−x) La (x) Zr 2 O 7 that have distinctive features making them compatible with the purpose of photocatalysis. Gd (2−x) La (x) Zr 2 O 7 are also distinguished from others by more characteristics and that assures the novelty of this work.
To understand deeply the fascinating features of pyrochlore, we must know about its classification and composition. Pyrochlore-structured oxides offer a wide range of compositions that lead to outstanding characteristics and applications in superconductivity, thermal barrier coatings, luminescence, and ferromagnetism. Pyrochlore has the general formula A 2 B 2 O 6 O (A = Y or rare earth; B = Ti, Zr, Hf, Sn, Tc, or Pb) 30 . The pyrochlore structure is a superstructure variant of the simple fluorite structure (AO 2 = A 4 O 8 , with the A and B cations arranged along the 110 direction). The extra anion vacancy is found in the tetrahedral interstice between two B-site cations. Geometrical frustration and new magnetic effects are particularly vulnerable in these systems. Electronic insulators (e.g., La 2 Zr 2 O 7 ), ionic conductors (e.g., Gd 1.9 Ca 0.1 Ti 2 O 6.9 ), mixed ionic and electronic conductors, spin ice systems (Dy 2 Ti 2 O 7 ), spin glass systems (Y 2 Mo 2 O 7 ), and Haldane chain systems (Tl 2 Ru 2 O 7 ) are among the physical properties of the pyrochlore structure (Cd 2 Re 2 O 7 ). More disordered structures, such as bismuth pyrochlores, have also been investigated due to their fascinating high-frequency dielectric properties.
Interestingly, herein, rare-earth zirconates with the formula Ln 2 Zr 2 O 7 (Ln = La-Gd) correspond to the pyrochlore structural type, which in turn belongs to the fluorite homologous series. The pyrochlore structure, as it is known, is a variant of the fluorite structure in which 1/8 of the oxygen is removed and oxygen vacancies are somewhat ordered [31][32][33] . Consequently, the effect of La 3+ -substituted Gd 2 Zr 2 O 7 on the structural parameters, crystallite size, microstructure, and optical properties was considered. Meanwhile, CV was selected as the target elicited organic contaminant model to test the photo-Fenton performance of La 3+ -substituted Gd 2 Zr 2 O 7 nanoparticles.

Materials and methods
Preparation of the Gd (2−x) La (x) Zr 2 O 7 nanoparticles. Materials used in the preparation process include gadolinium nitrate hexahydrate Gd(NO 3 ) 3 . 6H 2 O that was supplied from Alfa Aesar Co., as well as zirconyl oxychloride octahydrate ZrOCl 2 .8H 2 O from Alpha Chemika Co. and tartaric acid COOH(CHOH) 2 COOH from Lanxess Co.
The sol-gel auto-combustion technique will be used to create Gd (2−x) La (x) Zr 2 O 7 nanoparticles, where x equals (0.1, 02, 0.3, and 0.5). After mixing, ZrOCl 2 .8H 2 O and GdH 12 N 3 O 15 were dissolved in 40 ml of distilled water in a 1:1 molar ratio. Tartaric acid, which functions as a fuel, will be added to the mixture on a heater set to 350 °C until sol-gel forms. Following that. Sol-gel will be dried in an oven at 100 °C for 24 h. Finally, the dried sample will be fired in a furnace at 1100 °C for two hours to generate Gd (2−x) La (x) Zr 2 O 7 nanoparticles. Sample characterization. The phase composition, crystallinity, crystallite size, and structural parameters of the different samples with Cu K α radiation were established based on the X-ray diffractometer diffraction patterns (Philips PW 1390). Thermo Scientific's Nicolet iS 10 spectrophotometer is frequently used to measure the Fourier transform infrared (FTIR) absorption spectra of prepared samples at room temperature, which range from 4000 to 400 cm −1 . The tested sample was created using the compression procedure, which entailed mixing 200 mg of KBr and 2 mg of powdered Gd (2−x) La (x) Zr 2 O 7 samples to create a transparent disc. UV/Vis/Near IR spectrophotometry (a Jasco V770, Japan) technique is the technology that is used to investigate the optical properties and optical energy gap. The microstructure of Gd (2−x) La (x) Zr 2 O 7 particles was inspected using transmission electron microscopy (Jeol JEM-1011, Japan). www.nature.com/scientificreports/ Photoreactor and photocatalytic measurement. In the visible part of the electromagnetic spectrum, crystal violet has a large absorption band (591 nm). Despite its usage in the textile, culinary, and cosmetic sectors, CV dye can damage aquatic environments. As shown in Fig. 1., using Gd (2−x) La (x) Zr 2 O 7 nanoparticles as a model, it was chosen as the target organic pollutant to study how it degraded in the presence of UV radiation. The photocatalytic oxidation experiments were carried out in a 100 ml beaker mounted on a magnetic stirrer and exposed to two parallel, 36-Watt UV lamps above the beaker, which was housed in a lamina with silvered inside walls. To ensure uniform photocatalyst dispersion and facilitate the establishment of an adsorption-desorption equilibrium, the catalyst was introduced to a beaker containing 50 mL of an aqueous solution of CV (10 ppm). A solution of 33% H 2 O 2 in 1 mL was added to the reaction container. The reaction was thought to have started with the addition of H 2 O 2 . At regular intervals during the procedure, liquid aliquots were taken out of the vessel (15 min). Centrifuging was done on the liquid before analysis. The liquid solutions were analyzed using a visible spectrophotometer after the reactions. Based on the organic dye's rate of degradation, kinetic investigations were carried out. The degradation processes of the dye molecules might be expressed as follows using the Langmuir-Hinshelwood model, which adopts that dye degradation follows pseudo-first-order kinetics: Integrating this equation yields the following relationship, with the restriction that C = C o at t = 0, where C o is the initial concentration in the bulk solution after dark adsorption and t is the reaction time.
In this equation, Ao stands for the dye's initial absorbance, and T stands for the dye's absorbance at time t. t is the amount of time the dye was exposed to light, and Kapp is the apparent reaction rate constant. The slope of the linear plots on a graph of − ln[At/A 0 ] vs. time should be equal to the apparent-first-order rate constant (Kapp). The Kapp values show how quickly photocatalyst materials break down CV molecules when H 2 O 2 and visible light are present. The CV removal efficiency (η) was determined in the following manner: Beer-law Lambert states that the dye's absorbance is proportionate to its CV dye concentration.
Ethical approval. The submitted work is original and it is not been published elsewhere in any form or language (partially or in full). All authors agreed with the content that all gave explicit consent to submit, and that they obtained consent from the responsible authorities at the institute/organization where the work has been carried out. www.nature.com/scientificreports/ Results and discussion X-ray diffraction (XRD). X-ray diffraction pattern of Gd (2−x) La (x) Zr 2 O 7 fabricated based on a sol-gel autocombustion route with tartaric acid as a fuel and annealed at 1100 °C for 2 h is depicted in Fig. 2 The prepared rare earth oxide zirconate-based material is mainly. It has a pyrochlore or fluorite crystal structure. Pyrochlores generated from (La 1-x Gd x ) 2 Zr 2 O 7 , If the crystal structure is explained using La 2 Zr 2 O 7 as an example, La 3+ cation is in a specific crystallographic position, 16c (0, 0, 0) (A), Zr 4+ The cation occupies 16d sites (1/2, 1/2, 1/2) (B sites), On the (001)  where the wavelength of the Cu target is 1.5406 nm, d RX is the crystallite size, k = 0.9 is a correction factor to account for particle morphologies, FWHM is the full width at half maximum, and is the Bragg angle. As shown in Table 1 There are similar properties between La and Gd to be substituents for each other. The electronic structure for both of them respectively, [Xe] 5d 1 6s 2 and [Xe] 4f 7 5d 1 6s 2 . Atomic radii are respectively, 195 and 233 pm. Therefore, the results of crystallite size in nm and Lattice parameters for Gd (2−x) La (x) Zr 2 O 7 are very reasonable.
(4) d RX = k /β cos θ   Fig. 3. It should be observed that the absorption peak between wave numbers 1640 cm −1 and 3425 cm −1 is connected to the bending and vibrating of OH in water attributable to the water that has been absorbed from the atmosphere 34 Fig. 4. The grain growth rate increased with evaluated La 3+ ion concentration. The grain displays a cubic-like structure with a degree of agglomeration. The particle size was increased from 20 to 50 nm with a further increase in La 3+ ions. Such results can be explained on the basis of the change of ionic radius between  www.nature.com/scientificreports/ La 3+ and Gd 3+ ions. The SAED diffraction patterns indicated in Fig. 4 were in agreement with a defect-fluorite structured fluorite structure. Figure 5 indicates the Electron Diffraction pattern of Gd (2−x) La (x) Zr 2 O 7 nanoparticles which proves the results of the X-ray and both prove the crystalline features of the prepared samples.
Optical properties. Gd (2−x) La (x) Zr 2 O 7 phosphors' optical band gap is identical to that of Gd (2−x) La (x) Zr 2 O 7 itself based on the Kubelka-Munk theory and diffuse UV-Vis spectral reflectance Fig. 6 illustrates the sample's diffuse reflectance spectrum.
From the measurement of UV/Vis. analysis, it is noticed that an increase in the absorbance spectra of the contaminated water (crystal violet solutions) with the increase in purification time. The Kubelka-Munk (K-M) function is used to transform reflectance spectra to comparable absorption spectra as shown in Fig. 7.
The descriptions of the following equations, which were obtained from the literature for the Eg calculation, indicate the sort of transition that was taken into consideration.    www.nature.com/scientificreports/ According to the examination of the visible spectrophotometer measurements, the absorbance spectra of the contaminated water (crystal violet solutions) increase as the purification time increases. From the Crystal Violet solution, known concentrations of 1, 2, 3,…, and 10 ppm were generated. By using a visible spectrophotometer to measure their absorbance spectrum, a calibration curve for those concentrations was created, and an equation was then developed to use their absorbance to compute the unknown amounts that would follow from the purification procedure, as shown in Fig. 9.
It was put through a photocatalyst test using crystal violet solution (10 ppm) with intervals of 15 min between 0 and 90 min. Gd (2−x) La (x) Zr 2 O 7 nanoparticles were utilized as a catalyst for the Fenton-like reaction to purify by being exposed to hydrogen peroxide solution (32 watts). Different researches have been carried out with lots of materials for decolorization of crystal violet by photodegradation or even Fenton-like reaction as shown in Table 3 and Gd (2−x) La (x) Zr 2 O 7 nanoparticles proved a great efficiency compared with these materials.
It is worth noting that Gd (2−x) La (x) Zr 2 O 7 has never been used with the same composition in the photocatalysis process, according to our consideration. According to the literature survey, there are two types of researches or more that prepared the same elements used in our research, but with a different composition, the basis of which is La 2 Zr 2 O 7 , with substitutions of gadolinium, unlike our research in which G 2 Zr 2 O 7 is the base. In addition, this composition was not used in the process of water purification, and this indicates that our research is one of the first research that used Gd (2−x) La (x) Zr 2 O 7 with the same method in the photocatalysis process 37,38 .
The purification procedure was performed under identical conditions using gadolinium zirconium oxide alone, hydrogen peroxide alone again, and then both of them combined with 1 ml of hydrogen peroxide and 0.025 g of pyrochlore in 50 ml of crystal violet solution. It was observed that pyrochlore alone demonstrated a respectable efficiency and that this efficiency was increased by combining it with hydrogen peroxide. They showed rapid degradation of crystal violet, going from a concentration of 10 ppm to 0 ppm in no more than 90 min. By comparing with other researches, most of them used the same percent of photocatalyst in wastewater treatment process 39 . Table 2 shows the values of change in concentration or relative concentration (C/C 0 ) for crystal violet solutions (50 ml) during the purification process. The photocatalysis process has been carried out in three parallel categories. The first of them included the photocatalysis of crystal violet solution (50 ml) by using 1 ml of hydrogen peroxide alone (H 2 O 2 ). The second has proceeded with Gd (2−x) La (x) Zr 2 O 7 pyrochlore in addition to hydrogen peroxide. The last one was carried out by using Gd (2−x) La (x) Zr 2 O 7 pyrochlore alone (Gd (2−x) La (x) Zr 2 O 7 nanopowder). Moreover, all of these categories have proceeded under the influence of ultraviolet radiation in which the sample was taken for analysis every 15 min.
Photocatalysis efficiency is a vital parameter that should be taken into consideration. In the case of photocatalysis with Gd (2−x) La (x) Zr 2 O 7 nanoparticles alone, it proved an average efficiency according to values of relative concentration (C/C 0 ) in Table 2.
For the catalysis with pyrochlore in addition to hydrogen peroxide, the interpretation is slightly different, as it is done with a process called Fenton-Like Reaction, and in this case, hydrogen peroxide is indispensable. Similar to the situation of purification with hydrogen peroxide alone, this case is characterized by its high efficacy and a rate of removal that approaches zero in a period time that is likewise close to 90 min. Although it may appear superficially that Gd (2−x) La (x) Zr 2 O 7 pyrochlore has no function as long as we get the same percentage with or without it, the reality is rather different. The efficiency of the purification process in the case of pyrochlore with hydrogen peroxide is better than hydrogen peroxide alone, even if we get the same efficiency approximately at the end of the 90 min purification period according to the values of the relative concentrations 5 . Thus, It was found that the efficiency of the Fenton-Like approach, which comprises pyrochlore and hydrogen peroxide, is greater than pyrochlore alone at the start of the process. Furthermore, we employed a very low concentration of pyrochlore, up to 0.025 g per 50 ml of pollutant. If this percentage is increased, the purification process will be www.nature.com/scientificreports/ accelerated. and thus proves the efficiency of pyrochlore in the degradation process by employing a Fenton-like reaction (Table 3). In this context, the photocatalytic method is characterized by the separation of electron/hole pairs (e − /h + ) and the separation of photogenerated carriers can be connected to the crystalline phase, the aspect ratio of nanopowders, electronic structure of the facets, and defects. The reaction Eqs. (6)- (19) can be used to represent the mechanism of photocatalytic degradation of crystal violet dye 40 . As a result of the presence of hydroxyl groups, water, and oxygen at the surface of (Gd (2−x) La (x) Zr 2 O 7 nanoparticles, the electrons (e − ) in the conduction band react with species absorbed on the (Gd (2−x) La (x) Zr 2 O 7 surface, free radicals to generate O 2 ·− , and following the subsequent steps, it results in the generation of OH · radicals.
Finally, the produced agents degraded the dye pollution.
It is known that the use of H 2 O 2 in combination with the photocatalyst was shown to be effective in eliminating hazardous organic species from water. H 2 O 2 can hinder the surface recombination of electron-hole pairs and stimulate the production of hydroxyl radicals 5 .
Due to its reaction with the valence band holes and conduction band electrons in photocatalytic systems, H 2 O 2 raises the concentration of the · OH and · O 2 radicals. H 2 O 2 can react with · OH, HO 2 · , or · O 2 radicals to produce further radicals (Eqs. 20-25). The oxidative species interact with one another or with the holes they  The great differences are mainly derived from photo-induced thermal energy, which in-situ heating active sites to lower the H 2 O 2 activation barrier 44 .

Conclusion
In summary, Gd (2−x) La (x) Zr 2 O 7 with a variable Gd/La ratio was successfully prepared using a sol-gel auto-combustion approach based on tartaric acid as fuel. The La-substituted Gd 2 Zr 2 O 7 nanopowders existed in a defective fluorite phase. The crystallite size of the synthesized nanoparticles was found to be from 28.5 to 39.2 nm. Unit cell volume was found to be 1.2 × 10 -21 cm 3 . The grain size of Gd (2−x) La (x) Zr 2 O 7 increased with increasing La 3+ ion content. Most of the synthesized particles demonstrate a cubic-like structure. The diffuse reflectance spectrum confirmed the synthesized particles are highly transmissive. The band gap energy of the synthesized Gd (2−x) La (x) Zr 2 O 7 samples was found to be (4-3.6 eV). Finally, the synergistic effect of Gd (2−x) La (x) Zr 2 O 7 and H 2 O 2 possessed an advanced oxidation reaction with high efficiency towards organic pollutants degradation, such as crystal violet dye, in a cost-effective way and a short time.

Data availability
All data generated or analyzed during this study are included in this article.